In a bit of a surprise, the average size of some bird species has gotten bigger in central California over the last few decades. The finding runs counter to predictions that a warming climate would tend to cause creatures to downsize – and highlights the sometimes convoluted effects that climate change can have.

Between 1971 and 2010, birds banded at two sites near San Francisco Bay and the Point Reyes National Seashore.have gotten bigger in two ways, a research team reports in Global Change Biology: The birds’ wings have grown longer at a rate of 0.02% to 0.08% per year, and they’ve gotten, on average, heavier too.

The trend – distilled from measurements of nearly 33,000 birds — “completely surprised” San Francisco State University biologist Gretchen LeBuhn, who conducted the study with then-graduate student Rae Goodman. “It’s one of those moments where you ask, ‘what’s happening here?'”

The results were unexpected because a well-known ecological rule, called Bergmann’s Rule, states that animals tend to be larger in colder climates. In part, that’s because larger animals conserve body heat better. Under Bergmann’s Rule, some scientists have predicted that animals would get smaller as the Earth warmed up.

The California findings, however, show that the connection may not be so simple. That’s because climate change can affect body size in a variety of ways, the authors write. For instance, birds might get bigger as they store more fat to ride out severe weather events, which are expected to be more common under global climate change. Climate change could also alter a region’s plant growth, which may eventually lead to changes in a bird’s diet and affect its size. Other research has suggested that a more variable climate can also select for birds with bigger body sizes, perhaps because larger animals can ride out fluctuations better. After testing and discarding a number of other explanations, Goodman and her colleagues became confident that climate change was behind the changes they say. The birds may be responding to climate-related changes in plant growth or increased climate variability in central California, they suggest.

The findings offer a glimpse at the potent effects of climate change across a wide range of species, LeBuhn says. “Even over a pretty short period of time, we’ve documented changes in important traits like body size, where we don’t expect to see much flexibility.” And the results gave LeBuhn “a little more hope that these birds are able to respond — hopefully in time — to changes in climate.” — David Malakoff | November 1, 2011

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3 Comments

As someone who fully understands the reality of climate change and its potential impact, I don’t really like this type of evidence.

If science can explain any finding with “it’s caused by climate change”, regardless of what that finding is (bigger or smaller birds, in this case), that doesn’t seem like good science to me.

I understand that the earth is a massive dynamic system with unfathomable number of variables. However, if we can’t make a reasonable hypothesis and test it, I’m not sure we’re doing science.

In other words, if we say that climate change should produce smaller birds, then our findings surprise us and it’s producing larger birds, I’m not sure the reasonable conclusion is “climate change did this, but we just hadn’t thought about it enough before we developed a hypothesis”.

Interesting info but in the end the value or meaning is unknown. The researcher seems disappointed that he doesn’t have proof of yet another negative effect of global warming and then tries to readjust and offer a 180 degree hypothesis.

I have this picture of a guy weighing and re-weighing birds and each time he gets a heavy one thinking “Damn it!”

Without first reading the paper, I will be careful about what the authors actually found or inferred. But it is important to realize that this finding has quite a different meaning for “blaming climate change”, one that has more to do with basic scientific understanding of adaptation to environments. Supporting Bergmann’s rule–finding smaller birds–could not be construed as a “negative effect of climate change”. It would be an expected ADAPTIVE response of the birds, with tradeoffs still to be discovered. Instead, we find that body size is larger, which means a)we don’t fully understand what changes birds are experiencing and/or b)we need a better understanding of how temperatures actually select for larger vs smaller body size. Either one of these poses important questions, answers to which contribute to basic principles of development in relation to environments and also to better understanding of how climate change is experienced by organisms. Climate change provides a natural experiment as well as the challenge of uncontrollable change.